BOLD imaging has been used to monitor oxygenation in brain by capitalizing on the fact that deoxyhemoglobin has a paramagnetic susceptability and so changes in deoxyhemoglobin content can be visualized using MR imaging. This phenomenon forms the core of the field of cognative, or functional MR imaging. In addition, this measurement provides a rapid, spatially selective method of assessing the brain's response to acute hypoxia. We are investigating the relationships between changes in deoxyhemoglobin using BOLD imaging and brain pO2 using EPR oximetry. The relationships between tissue pO2 values and averaged hemoglobin saturation are not easily predicted and must be measured. With the combination of "non-invasive" EPR oximetry and MR imaging, we hope to calibrate the BOLD response in living brain, determine the regional senstivity of brain to functional imaging and identify spatial heterogeneity in the brain's response to acute hypoxia.